In process measurement and automation technology, for measuring such physical parameters of a fluid flowing in a pipeline as the flow rate, density and/or viscosity, measuring instruments are often used of the type that by means of a measurement pickup of the vibrational type through which the fluid flows and a measurement and control circuit connected to the measurement pickup, bring about reaction forces in the fluid, such as Coriolis forces that correspond to the flow rate, inertial forces that correspond to the density, and/or frictional forces that correspond to the viscosity, and so forth, and that on the basis of them generate a measurement signal that represents the flow rate and/or a measurement signal that represents density of the fluid.
Such measurement pickups of the vibrational type are described for instance in International Patent Disclosures WO-A 03/021202, WO-A 03/021203, WO-A 01/33174, WO-A 00/57141, and WO-A 98/07009, U.S. Pat. Nos. 5,796,011, 5,301,557, and 4,876,898, and European Patent Disclosures EP-A 553 939, EP-A 1 001 254, and EP-A 1 154 243. For carrying the fluid, the measurement pickups each include at least one hollow body, embodied as a measurement pipe and mounted so it is capable of vibrating, with a pipe segment that to generate the aforementioned reaction forces is made to vibrate in operation, driven by an electromechanical exciter assembly. For detecting vibration, particularly on the inlet and outlet sides, of the pipe segment, the measurement pickups moreover each have a sensor assembly that reacts to motions of the pipe segment.
In measurement pickups of the type described, the measurement pipes and the exciter and sensor assemblies are typically disposed inside a further hollow body, which is embodied as a pickup housing and serves to mount the measurement pipe. The pickup housing serves not only to mount the at least one measurement pipe but also and in particular to protect it along with the exciter and sensor assemblies as well as other components located on the inside of the measurement pickup from external environmental factors, such as dust or splashing water. Examples of such pickup housings are shown for instance in WO-A 03/021202, WO-A 03/021203, WO-A 01/33174, and WO-A 00/57141, U.S. Pat. No. 5,301,557, and European Patent Disclosures EP-A 1 001 254, and EP-A 1 154 243. Often, the user also demands of such pickup housings that, in the event that the pipe segment bursts, they withstand the then usually markedly increased internal pressure without leaking for at least a predetermined length of time; in this respect, see also WO-A 03/021203, and WO-A 00/57141, U.S. Pat. Nos. 6,044,715, and 5,301,557, or European Patent Disclosure EP-A 1 001 254. At least for applications with fluids that are toxic or that readily ignite, the housing must in some cases also be capable of meeting the demands to be made of a safety vessel.
Increasingly, the development of measurement pickups has also led to the capability of markedly reducing the mass of the measurement pickups. Especially with the pickup housings as well, the result was that in modern measurement pickups, despite the high pressure resistance required, they usually have a lesser wall thickness of less than 5 mm, associated with a comparatively low mass. However, reducing the mass of the pickup housing can sometimes mean that the mechanical natural frequencies of the measurement pipes and pickup housings used differ from one another less and less, which in turn impairs the measurement accuracy of the measuring instrument. One possible way of reducing this effect is for instance, as also proposed in WO-A 01/33174, to fix additional masses on the pickup housing in order to achieve a targeted mistuning relative to the measurement pipe or the measurement pipes.
One disadvantage of such an embodiment, however, is considered to be above all that it requires additional structural provisions, which involve additional expense in producing the measurement pickup. Moreover, mounting such additional masses can also cause the pickup housing to have many new vibration modes, sometimes with natural frequencies that are problematic in other ways, for instance with respect to the connected pipeline.